1. Field of the Invention
The present invention generally relates to a method of fabricating a silicon substrate having a thin buried silicon oxide layer (SOI substrate) and in particular to a method of this sort which makes it possible to produce an SOI substrate having a buried silicon oxide layer that is extremely thin and has excellent uniformity.
2. Description of the Related Art
The known techniques of fabricating an SOI substrate all have a number of drawbacks, in particular, a low production yield, a substrate quality which is still inadequate, and the production of a thin Si layer and a thin buried silicon oxide layer which are relatively thick and of mediocre uniformity.
A first method of fabricating an SOI substrate, known by the name of xe2x80x9cSIMOX technologyxe2x80x9d, consists of forming the buried SiO2 layer in a silicon substrate by implanting oxygen at high dose followed by annealing at a temperature greater than 1300xc2x0 C. A major drawback of this method is that it requires nonstandard equipment. Furthermore, the length of time for the process of implanting oxygen at high dose considerably reduces the production efficiency.
The substrates obtained by this method also suffer from inadequate quality of the buried silicon oxide layer and of the thin silicon layer (high pinhole density).
Finally, because the thin layers (thin Si layer and thin buried silicon oxide layer) are determined by the implantation process, this method makes it difficult to achieve thicknesses of less than 50 nm for the thin silicon layer and 80 nm for the buried SiO2 layer.
A second method, known by the name of xe2x80x9cBESOI techniquexe2x80x9d, consists of forming a thin SiO2 film on a surface of a first silicon body, then bonding this first body to a second silicon body by means of the thin SiO2 film, and finally, removing part of one of the silicon bodies by mechanical grinding and polishing in order to form the thin silicon layer above the buried silicon oxide layer.
The silicon oxide layer on the first silicon body is formed by successively oxidizing the surface of this first body, then etching the oxide layer formed in order to obtain the desired thickness.
This method only allows relatively thick buried silicon oxide layers and silicon layers to be produced on the buried silicon oxide because of the poor control of the etching method. Furthermore, the thin layers obtained by this method have poor uniformity.
A third method, known by the name of xe2x80x9cSMART CUT technologyxe2x80x9d, consists of forming, by oxidation, a thin silicon oxide layer on a first silicon body, then implanting H+ ions in the first silicon body in order to form a cavity plane in this first silicon body under the thin silicon oxide layer. Subsequently, by means of the thin silicon oxide layer, this first body is bonded to a second silicon body and then the assembly subjected to thermal activation in order to transform the cavity plane into a cleaving plane. This makes it possible to recover, on the one hand, an SOI substrate and, on the other hand, a reuseable silicon body.
This method requires the implantation of a high dose of hydrogen atoms. In spite of using atoms of smaller size for the implantation, the surface of the thin silicon layer obtained is also damaged. Furthermore, since the thickness of the thin silicon layer is defined by the implantation energy of the hydrogen atoms, it is difficult to make this thickness less than about 50 nm.
The methods above are described, in particular, in the article SOI: Materials to Systems, A. J. Auberton-Hervxc3xa9, 1996, IEEE.
Therefore, a method of fabricating an SOI substrate, which overcomes the drawbacks of the methods of the prior art, may be desired.
In particular, a method of fabricating an SOI substrate which makes it possible to produce silicon oxide layers and silicon layers on the buried oxide layer that are very thin and of very good uniformity may be favorable.
A method of fabricating an SOI substrate that can be implemented in standard equipment may also be desired.
According to a first embodiment of the invention, the method of fabricating a silicon substrate having a thin buried silicon oxide layer includes:
a) the production of a first element having a silicon body, a main surface of which is coated with a buffer layer made of germanium or of a germanium-silicon alloy and with a thin silicon layer, in that order;
b) the production of a second element, distinct from the first element, having a silicon body, a main surface of which is coated with a thin silicon oxide layer;
c) the bonding of the first element to the second element such that the thin silicon layer of the first element is in contact with the thin silicon oxide layer of the second element; and
d) the removal of the buffer layer made of germanium or of a germanium-silicon alloy in order to recover the silicon substrate having a thin buried silicon oxide layer, on the one hand, and a reuseable silicon substrate, on the other hand.
According to a second embodiment of the invention, the method of fabricating a silicon substrate having a thin buried silicon oxide layer includes:
a) the production of a first element having a silicon body with a main surface and coated with a buffer layer made of germanium or of a germanium-silicon alloy, a thin silicon layer and a thin silicon oxide layer, in that order;
b) the production of a second element having a silicon body, a main surface of which is coated with a thin silicon oxide layer;
c) the bonding of the first element to the second element, such that the thin silicon oxide layer of the first element is in contact with the thin silicon oxide layer of the second element; and
d) the removal of the buffer layer in order to recover the silicon substrate having a thin buried silicon oxide layer, on the one hand, and a reuseable silicon substrate, on the other hand.
In a third embodiment of the invention, the method of fabricating a silicon substrate having a thin buried silicon oxide layer includes:
a) the production of a first element having a silicon body, a main surface of which is coated with a buffer layer made of germanium or of a germanium-silicon alloy, a thin silicon layer and a thin silicon oxide layer, in that order;
b) the production of a second element comprising a silicon body;
c) the bonding of the first element to the second element such that the thin silicon oxide layer of the first element is in contact with the silicon body of the second element; and
d) the removal of the buffer layer in order to recover the silicon substrate having a thin buried silicon oxide layer, on the one hand, and a reuseable silicon substrate, on the other hand.
The thin buffer layers made of Ge and germanium-silicon alloys of the first element may be produced by epitaxial deposition. For example, the thin buffer layers are produced by well-known methods of vapor deposition or of molecular beam epitaxial deposition, such that the thin silicon layer may have a very small thickness of a few nanometers while still having a suitable uniformity, typically from 1 to 50 nm.
The thin buried oxide layer may be produced by a known thermal oxidation method, such that it is possible to obtain an oxide layer of very high quality with a virtually arbitrary thickness that can vary from 2 nm to 400 nm.
As is known per se, the bonding of the first and of the second elements can be carried out by using the Van der Waals forces. In order to increase the strength at the interface between the elements, the assembly can possibly be subjected to annealing.
The buffer layer of the first element may consist of pure germanium, of a silicon-germanium alloy or of a silicon-germanium alloy containing carbon. More particularly, Si1xe2x88x92xGex alloys (0 less than x less than 1) or Si1xe2x88x92xxe2x88x92yGexCy alloys (0 less than xxe2x89xa60.95 and 0 less than yxe2x89xa60.05) can be used.
As is known, the germanium and the Si1xe2x88x92xGex and Si1xe2x88x92xxe2x88x92yGexCy alloys have very high selectivity to chemical etching by solutions or to anisotropic plasma etching. In the case of silicon-germanium alloys, in order to obtain etching of high selectivity, it is preferable that the proportion of germanium in the alloy is at least equal to 10% by weight, and preferably equal to or greater than 30% by weight.
The use of the silicon-germanium alloy containing a small proportion of carbon makes it possible to reduce the high stresses between the silicon layers and the buffer layer. Thus, it is possible to use alloys with higher germanium concentrations and, consequently, with better etching selectivity, while producing a relaxation of the stresses. It is also possible to form a buffer layer made of a silicon-germanium alloy having a germanium concentration gradient (relaxed SiGe buffer layer). In this type of buffer layer, the germanium concentration increases from the silicon body of the first element.
It is thus possible to make relatively thick buffer layers, which can significantly increase the etching rate. Because of their thickness, the buffer layers may have a surface free of dislocations (all the dislocations and defects being located in the low part of the buffer layer), which provides good conditions and good continuity for the epitaxial growth of the thin single-crystal silicon layer which is to be deposited later.
In the SiGe alloy layer, the concentration profile of the germanium can be nonuniform. Thus, at the bottom of the layer, the Ge concentration can be 0%, then increase to 50% (or 70% and even 100%), and then decrease down to 0%. This solution makes it possible to avoid deformations in the upper silicon layer and to remove the thickness limitations. A high molar fraction of Ge in the SiGe alloy (in particular in the middle of the layer) provides, on the one hand, a very high selectivity of the etching method and, on the other hand, freedom from the risk of relaxation of the Si film.